1. Field
The embodiments described below relate generally to the delivery of therapeutic radiation to a patient. More specifically, some embodiments are directed to the positioning of a patient volume prior to the delivery of radiation therapy.
2. Description
According to conventional radiation therapy, a beam of radiation is directed toward a target volume (e.g., a cancerous tumor) located within a patient. The radiation beam delivers a predetermined dose of therapeutic radiation to the target volume according to an established treatment plan. The delivered radiation kills cells of the target volume by causing ionizations within the cells or other radiation-induced cell damage.
Treatment plans are designed to deliver a particular radiation dose to a target volume, while ensuring that surrounding healthy tissue does not receive an unsafe dose. To design a radiation treatment plan, a designer views a three-dimensional image of a patient volume (i.e., a “planning” image) and defines one or more treatment beams to be delivered to the volume by a linear accelerator (linac), with each beam having a delivery angle, a shape, a dose rate and other characteristics. The designer also defines a treatment isocenter within the volume through which the treatment beams pass.
A treatment plan therefore assumes that relevant portions of a patient will be in particular positions relative to a linac during delivery of the treatment radiation. Therefore, the goals of the treatment plan may not be achieved if the relevant portions are not positioned in accordance with the treatment plan during actual delivery of the radiation. More specifically, errors in positioning the patient can cause the delivery of low radiation doses to tumors and high radiation doses to sensitive healthy tissue. The potential for misdelivery increases with increased positioning errors.
Many techniques have been developed to verify patient positioning prior to the delivery of treatment radiation by a linac. Specifically, this verification is intended to confirm that the treatment isocenter of the planning image is aligned with the isocenter of the linac.
According to one currently-used positioning system, tattoo marks are placed on a patient during acquisition of the planning image. The tattoo marks provide an external indication of the location of the treatment isocenter as defined by the treatment plan. Prior to treatment, radio-opaque seeds are placed on the tattoo marks and another three-dimensional image (i.e., a “pre-treatment image”) is acquired, in which the seeds are visible.
The patient is then positioned for treatment by aligning the seeds/tattoos with lasers inside the linac treatment room. The lasers define the linac isocenter, so, by referring to the locations of the seeds in the pre-treatment image, it is possible to identify the linac isocenter within the pre-treatment image. Rigid registration between the planning image and the pre-treatment image provides the location of the treatment isocenter within the pre-treatment image, thereby allowing computation of the offset between the linac isocenter and the treatment isocenter. The patient is moved according to this offset and treatment commences.
The foregoing process presents several problems. First, the tasks of accurately securing the seeds on the tattoo marks and aligning the seeds with the in-room lasers are time-consuming. Moreover, both of these tasks are subject to human error. Accuracy of the process also depends on precise calibration between the in-room lasers might and the linac isocenter.
Improved patient positioning and verification systems are desired.